1. Field of the Invention
The present invention relates to a sensor arrangement as indicated in the introduction of claim 1, for use on the seabed. Such sensor arrangements may be used, permanently or temporarily installed on the seabed, for surveillance of petroleum reservoirs and adjacent well systems.
The invention also relates to a method for the installation and moving of seismic sensor arrangements on the seabed.
2. Description of Related Art
To be able to perform seismic surveys of formations below the sea, a plurality of sensor nodes are placed on the seabed, preferably in a regular disposition. These sensor nodes must be in contact with a solid medium, leading to requirements for the deployment operation. From Norwegian Patent Application No. 20025896, an arrangement is known, in which to each sensor node is connected a remotely operated control and registration unit for telemetric transmission of data to a control center, which is arranged in an above-water receiver.
In another embodiment, on the same basis, the registered data is transferred by either picking out the storage medium physically, or by transferring the data on the storage medium through a temporary, externally connected cable connection. The sensor arrangement is run by an internal battery, and the time reference for the seismic measurements is an internally arranged clock, with very good long term stability.
For the deployment of a plurality of such sensor nodes, each sensor node is provided with a handle or a bracket that can be grasped by the gripping means of a remotely operated vehicle (ROV) or a remotely operated tool (ROT). Each sensor node, with appurtentant control and data registration unit (CDU), is then lowered down onto the seabed with a crane. In good conditions, the surface vessel can control the position of the deployment with an accuracy of within a few meters. An ROV is then used, primarily for moving the sensor node from a parked position on the CDU to a free position on the seabed. The ROV may possibly be used to correct the position of the sensor arrangement, and to release the crane attachment. In this operation, the ROV may be used to arrange the sensor node in the correct position. This solution may function well for installations with a limited time duration, in the order of one month, but it becomes less feasible when substantially longer installation times are required. Typical areas of application for such seismic sensor installations may be the monitoring of oil and gas reservoirs over time (e.g. 10–15 years), including well paths during drilling and operation.
An available method for realizing such a “permanent” seismic sensor installation is to dig one or more sensor cables down into the seabed. Via a system of connection cables, the cable installation is provided with time reference, instructions, and energy. Collected data from each individual registration unit in the cable is led through common lines, up to a centrally placed data storage medium, normally placed above the sea surface. The embedment process is necessary to achieve good acoustic coupling between sensors and seabed, prevent unwanted relocation due to hydrodynamic influence, as well as to protect the cable against damages from, for instance, trawling or any other external influence. The embedment process typically consists of forming kilometers of ditches in the seabed, into which the cable strings are placed. This may represent substantial and lasting environmental interventions in the seabed biotope, and require the use of large, energy consuming, and air pollutive equipment during the embedment process. The cost associated with the embedment will typically be in the same order of magnitude as the value of the entire cable installation. The cables cannot be laid over existing obstructions on the seabed or in areas where future underwater installations are planned. Furthermore, it turns out that in practice, it is difficult to maintain/repair the buried parts of the cable system without having to dig them out again. A cable system which is not maintained will typically detonate within a few years due to electric failure and water intrusion. Depending on the type of failure in the cable, the failure may have consequences for parts of the cable that are downstream in relation to the position of the failure. If one, on the other hand, moves a cable to the surface for maintenance and repairs, it will be a challenge replacing it later in the exactly same position as before, and with the same acoustic coupling to the seabed as the original coupling. Both factors are particularly important, for instance, for observing small changes in the reservoir characteristics through time. It will also imply substantial risk of damaging cables that must be raised up again.
From US patent publication U.S. Pat. No. 6,474,254 (WesternGeco, 2002) it is known to deploy cables with attached sensors in a network on the seabed, but this requires the application of a remotely operated vehicle (ROV), and provides no possibility for the repair of individual sensors.
From NO patent publication 168622 (Den norske stats oljeselskap, 1991), a seismic cable arrangement is known, where a series of torpedo-shaped sensor arrangements are attached to a signal cable. These arrangements are deployed in a pattern on the seabed by means of a remotely operated vehicle (ROV). This solution has not proven to be satisfactory from a deployment and application point of view.
From NO Patent Application 20025831 (Institute Francais du Petrole, 2003) a system for seismic data collection is known, where sensor arrangements are placed onto the seabed by releasing them, allowing them to free fall, resulting in torpedo-shaped penetration down into the seabed. This equipment does not satisfactorily provide the possibility for the placement in a substantially permanent network of sensor arrangements with a cable connection to the individual units. Neither does this provide satisfactory possibilities for maintenance and repairs.